Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps and backlights. The technology relies upon thin-film layers of materials coated upon a substrate. However, as is well known, much of the light output from the light-emissive layer in the OLED is absorbed within the device. Because light is emitted in all directions from the internal layers of the OLED, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
OLED devices generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic electroluminescent (EL) element, and a cathode. The organic EL element disposed between the anode and the cathode commonly includes one or more of an organic hole-injection layer (HIL), an organic hole-transporting layer (HTL), an emissive layer (EML), an organic electron-transporting layer (ETL) and an organic electron-injection layer (EIL). Holes and electrons recombine and emit light in the EML layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer and the hole transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EML can determine how efficiently the electrons and holes recombine and result in the emission of light, etc. It has been found, however, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. In most cases, these trapped photons are eventually absorbed, never leaving the OLED device, although some of these photons may escape from the edge of the device. In either case, they make no contribution to the useful light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent (or semitransparent) cathode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. Using ray optics, it has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
Referring to FIG. 2, a top-emitting OLED device as suggested by the prior art is illustrated having a substrate 10 (either reflective, transparent, or opaque). Over the substrate 10, a semiconducting layer is formed providing thin-film electronic components 30 for driving an OLED. An interlayer insulating and planarizing layer 32 is formed over the thin-film electronic components 30 and a first patterned reflective electrode 13 defining OLED light-emissive elements is formed over the insulating layer 32. An inter-pixel insulating film 34 separates the elements of the patterned reflective electrode 13. One or more first layers 14 of organic materials described above, one of which emits light, are formed over the patterned reflective electrode 13. A transparent second electrode 16 is formed over the one or more first layers 14 of organic material. A gap 18 separates the transparent second electrode 16 from an encapsulating cover 20. The encapsulating cover 20 is transparent and may be coated directly over the transparent electrode 16 so that no gap 18 exists. In some prior-art embodiments, the first reflective electrode 13 may instead be at least partially transparent and/or light absorbing. In a bottom-emitter embodiment, the substrate is transparent and the positions of the transparent and reflective electrodes are reversed. Typically, the reflective metal electrode 12 comprises Al, Ag, Mg, or alloys of these or other reflective metals. The transparent electrode 16 may comprise ITO or other transparent and conductive metal oxides. A semitransparent electrode, e.g. formed from thin metal layers, such as Ag, may also form all or part of this electrode.
As shown in a simpler form in FIG. 6 (for a top-emitter embodiment) and FIG. 7 (for a bottom-emitter embodiment), light emitted from one of the organic material layers 14 can be emitted directly out of the device, through the substrate 10 or cover 20, as illustrated with light ray 1. If the gap 18 either does not exist or is filled with a material whose optical index matches that of the cover or substrate, light may also be emitted and internally guided in the substrate 10 or cover 20 and organic layers 14, as illustrated with light ray 2. Alternatively, light may be emitted and internally guided in the organic layers 14 and electrode 16, as illustrated with light ray 3. Light rays 4 emitted toward the reflective electrode 13 are reflected by the reflective electrode 13 toward the substrate 10 or cover 20 and then follow one of the light ray paths 1, 2, or 3. Light emitted in these paths may be termed Modes I, II, and III light respectively.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. In particular, scattering layers employed in optical contact with the OLED layers can disrupt total internal reflection of light within the OLED device and increase the amount of light emitted from an OLED device. However, scattering techniques, by themselves, cause light to pass through the light-absorbing material layers multiple times where they can be absorbed and converted to heat. Moreover, trapped light may propagate a considerable distance horizontally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pixelated applications such as displays. For example, as illustrated in FIG. 8, a prior-art pixelated top-emitting OLED device may include a plurality of independently controlled pixels 50, 52, 54, 56, and 58 and a scattering layer 22 located between the cover 20 and the transparent electrode 16. A light ray 5 emitted from the light-emitting layer may be scattered multiple times by scattering layer 22, while traveling through the cover 20, organic layer(s) 14, and transparent electrode 16 before it is emitted from the device. When the light ray 5 is finally emitted from the device, the light ray 5 has traveled a considerable distance through the various device layers from the original pixel 50 location where it originated to a remote pixel 58 where it is emitted, thus reducing sharpness. Most of the lateral travel occurs in the cover 20, because that is by far the thickest layer in the package. (Note that the layer thicknesses in this figure have not been drawn to scale since the thickness differences of the various layers is too great to permit depiction to scale.) Also, the amount of light emitted is reduced due to absorption of light in the various layers. If the light-scattering layer is alternatively placed adjacent to the substrate 10 of a prior-art bottom-emitting device as illustrated in FIG. 9, the light may similarly travel a significant distance in the substrate 10 before being emitted.
U.S. Patent Application Publication No. 2004/0061136 entitled “Organic light emitting device having enhanced light extraction efficiency” by Tyan et al., describes an enhanced light extraction OLED device that includes a light scattering layer. Two transparent electrodes may be employed, along with the use of a reflective layer, such as a metal layer, behind one of the transparent electrodes. In certain embodiments, a low index isolation layer (having an optical index substantially lower than that of the organic electroluminescent element) is employed adjacent to a reflective layer in combination with the light scattering layer to prevent high angle (with respect to the normal) light from striking the reflective layer, and thereby minimize absorption losses due to multiple reflections from the reflective layer. The particular arrangements, however, may still result in reduced sharpness of the device and incomplete light extraction.
Referring to FIG. 10, e.g., the sharpness of a bottom-emitting active matrix OLED device employing a light-scattering layer coated on the substrate is illustrated. The average MTF (sharpness) of the device (in both horizontal and vertical directions) is plotted for an OLED device with the light-scattering layer and without the light-scattering layer. As is shown, the device with the light-scattering layer is much less sharp than the device without the light-scattering layer, although more light was extracted (not shown) from the OLED device with the light-scattering layer. FIG. 10 thus illustrates the reduction in sharpness that occurs when scattering layers are employed as taught in the prior art.
A variety of means for increasing the light output from OLED devices have been proposed. One such technique relies upon forming an optical cavity to increase light output and to control the color of the light output. For example, U.S. Pat. No. 6,737,800B1 describes a multicolor organic light-emitting display having an array of pixels having at least two different colors including a substrate; a reflective layer disposed over the substrate; and a first transparent electrode disposed over the reflective layer. The display also includes a second transparent electrode spaced from the first transparent electrode and an organic EL media disposed between the first and second transparent electrodes and arranged to produce white light. The display further includes at least first and second filters of different colors disposed respectively over different predetermined pixels of the array, and wherein the thickness of the first transparent electrode is separately adjusted for each different color to cause a substantial amount of the reflected component of colored light corresponding to its associated color filter to constructively interfere with a substantial amount of the non-reflected component of colored light corresponding to its associated color filter. However, such OLED designs suffer from manufacturing tolerance difficulties and the color of the light emitted from the device generally depends quite strongly on the angle of emission.
To simultaneously increase the amount of light output from an OLED device and preserve the sharpness and color of a pixellated OLED display device at a variety of viewing angles, co-pending, commonly assigned U.S. Ser. No. 11/065,082, filed Feb. 24, 2005, the disclosure of which is hereby incorporated by reference, describes the use of a scattering layer in combination with a transparent low-index element having a refractive index lower than the refractive index of the encapsulating cover or substrate through which light is emitted from the OLED device and lower than the refractive index range of the OLED element materials. Note that use of such a design is incompatible with optical cavity designs, however, as the scattering layer would destroy the constructive interference upon which such optical cavities rely.
Even the combined use of scattering and low-index layers, however, does not result in the emission of all of the light produced in OLED devices employing a conventional metallic reflective electrode. An electro-dynamic physical and optical model created by applicants demonstrates that the emission of light in the OLED structure employing scattering layers still results in considerable light being lost in the metallic reflective electrode through the formation of surface plasmons. An attempt to extract surface plasmons from an OLED device in the form of useful light is described in granted U.S. Pat. No. 6,670,772. However, the solution proposed requires very precise manufacturing tolerances and creates a very strong angular dependence on the color of light emitted.
There is a need therefore for an improved organic light-emitting diode device structure that avoids the problems noted above and improves the efficiency and sharpness of the device.